The United States submitted a new North Korean sanctions resolution to the UN Security Council, toning down its demands less than 24 hours before a vote due Monday, diplomats said, as it sought to bring China and Russia on board.

Washington has led the international drive to punish the rogue state after it detonated a nuclear device earlier this month.

The US had originally pushed for a strict oil embargo, as well as a freeze on the assets of North Korean leader Kim Jong-Un.

But late Sunday, diplomats said the asset freeze had been dropped from the new draft, and it now foresaw a progressive tightening of the oil taps, instead of something sudden and complete.

Among other concessions the new text also softens proposed restrictions on North Koreans working overseas, and on the inspection by force of ships suspected of carrying cargo prohibited by the UN.

Of five key original measures, a ban on textile exports from North Korea remained.

Britain and France -- permanent Security Council members along with the US, China and Russia -- have given Washington their unequivocal backing.

The sticking point will be opposition from the Security Council's two other veto-wielding permanent members, Russia and China.

The North's two main backers, they are wary of anything that might force the collapse of the regime and the resulting exodus of refugees.

The US Energy Information Administration (EIA) says estimates suggest Pyongyang imports about 10,000 barrels of crude oil a day, almost all of it from China.

In addition, according to figures from the International Trade Centre, a joint World Trade Organisation-United Nations agency, the North imported $115 million-worth of refined oil products -- which could include petrol and aircraft fuel -- from China last year. Another $1.7 million-worth came from Russia.

"We have been clear in close consultation with the Americans that oil has to be included as an element of sanctions," South Korean foreign minister Kang Kyung-Wha told reporters.

Whatever final text was adopted, she hoped it would "have significant consequences in terms of greater economic pressure on North Korea".

Washington has dangled the prospect of military action in the North, and threatened to cut economic ties with countries that continue to trade with the it -- around 90 percent of the North's external commerce is with China.

Kim Hyun-Wook, professor at the Seoul government's Korea National Diplomatic Academy, told AFP that Washington had softened its original "very strong draft proposal" because it was vital to keep Moscow and Beijing on board.

"It is only possible to criticise and rebuke China and Russia for not enforcing the sanctions if they vote for it at the UN Security Council," he said.

'Pay the price'

Early Monday, North Korea said it would not accept any chastisement over its nuclear and missile programme, which it says is vital to stave off the threat of an American invasion.

If Washington does "rig up the illegal and unlawful 'resolution' on harsher sanctions, the DPRK shall make absolutely sure that the US pays due price," its foreign ministry said, in a statement published by the official KCNA news agency.

The North has a long history of making florid threats against Washington and its allies without following through on them.

"The forthcoming measures to be taken by the DPRK will cause the US the greatest pain and suffering it had ever gone through in its entire history," the ministry said.

"The world will witness how the DPRK tames the US gangsters by taking (a) series of action tougher than they have ever envisaged."

Pyongyang has staged a series of missile tests in recent months, culminating in an intercontinental ballistic missile that appeared to bring much of the US mainland into range -- ramping up tensions and earning itself a seventh set of UN Security Council sanctions.

It followed up with a sixth nuclear test on September 3, its largest to date, which it said was a hydrogen bomb small enough to fit onto a missile and prompted global outrage.

Monday's expected vote is seen as a key test of resolve for the council, which united last month to adopt a resolution intended to reduce the impoverished country's export earnings by up to a billion dollars.

Chinese foreign ministry spokesman Geng Shuang told reporters that Beijing "approves further reactions and necessary measures by the UN Security Council" in response to the nuclear test.

He declined to be drawn on whether China was behind the weakening of the draft, saying it hoped the decision would be made "on the basis of full consultation and consensus".